Vulnerability Note VU#32650

Denial of Service Attack in NetBIOS Services

Overview

The NetBIOS Name Service (NBNS) provides a means for hostname and address mapping on a NetBIOS-aware network. The NetBIOS over TCP/IP protocols (including NBNS) are described in the Internet Engineering Task Force (IETF) Request for Comments RFC1001 and RFC1002. These protocols do not specify a method for authenticating communications, and as such, machines running NetBIOS services are vulnerable to spoofing attacks.

NetBIOS is a set of defined software interfaces for vendor-independent PC networking and is primarily used on Microsoft Windows computers. NetBIOS is enabled by default on Windows95 and Windows98 machines.

Description

An attacker sending spoofed "Name Release" or "Name Conflict" messages to a victim machine could force the victim to remove its own (legitimate) name from its name table and not respond to (or initiate) other NetBIOS requests. This renders the victim unable to communicate with other NetBIOS hosts, thus resulting in a denial-of-service attack.

Impact

An attacker can cause a victim's machine to refuse all NetBIOS network traffic, resulting in a denial of service.

Solution

Block NetBIOS services at the the network perimeter. NetBIOS services include

NetBIOS Name Service, 137/tcp and 137/udp

NetBIOS Datagram Service, 138/tcp and 138/udp

NetBIOS Session Service, 139/tcp and 139/udp

Note that this prevents external hosts from sending NetBIOS Name Service traffic to internal machines, but it does not prevent local users from exploiting this vulnerability.

Furthermore, the CERT/CC recommends that sites, even those which do not use NetBIOS services, block all ports unless they are explicitly needed.

Note that no patch is being furnished for Win9x systems; Microsoft has publicly stated that patching these systems to disable name conflict resolution would cause more problems than it would help prevent, especially in networks with large numbers of Win9x systems.

Use IPSec to help secure internal IP communication. Information about configuring IPSec in Windows 2000 can be found at